Method and apparatus for investigating interfiber friction

ABSTRACT

Interfiber friction information is obtained by measuring the energy dissipated when a fibrous assembly is loaded in shear. The specimen of fibers is mounted between parallel surfaces one of which is connected to a pendulum so as to be oscillated in a direction parallel to the surfaces. During relative oscillatory movement between the surfaces, fibers of the assembly move relative to each other and the work expended in overcoming friction is reflected in the decay in the amplitude of the pendulum oscillations.

[151 Mesuse liiertell [54] MiE'iiIi-IHDD AND AiPlWtrliilA'iiiU 2,032,2022/1936 Dennis.........................................73/7

HiF iEiUi 3,087,326 4/1963 MacDonnell.....

Primary Examiner-Richard C. Queisser Assistant Examiner-John Whalen [72]lnventor: Kenneth 1L. liliertel, Knoxville, Tenn.

Attorney-Burns, Doene, Sweclcer & Mathis [57] AiliS'llllR/tt'l'lllnterfiber friction information is obtained by measuring the [22] Filed:

[21] Appl. No.:

energy dissipated when a fibrous assembly is loaded in shear.

The specimen of fibers is mounted between parallel surfaces one of whichis connected to a pendulum so as to be oscillated in a directionparallel to the surfaces. During relative oscillatory movement betweenthe surfaces, fibers of the assembly move relative to each other and thework expended in over- [52] US. [51] lint. [58] Field ofiieareh comingfriction is reflected in the decay in the amplitude of the pendulumoscillations.

[56] References Cited 7 UNTTED STATES PATENTS Claims, Drawing Figures2,296,657 9/1942 Wallace......................................,73/9

PAIENIEmwzz 1972 3,643 .490

sum 1 BF 2 ENTOR KENNETH L HERTEL BY /W a a 1.4

ATTORNEYS PAIENIEDFEB22 I972 SHEET 2 UF 3 SQUARE OF THE DIFFERENCEBETWEEN SUCCESSIVE AMPLITUDES X X/ am .w INVENTOR KENNETH L. HERTELATTORNEYS" METHOD AND APPARATUS EUR INVESTIGATING INTEIRTIIEERlFItlICTllUN BACKGRUUND OF THE INVENTION This invention relates to amethod and apparatus for determining interfiber friction properties andmore particularly to a method and apparatus for measuring the shearfriction of an assembly of fibers.

For most solid bodies, friction properties can be dealt withsatisfactorily in terms of coefficient of friction," defined as theratio between the tangential force required to produce sliding of onesurface upon another and the normal force pressing the two surfaces intocontact. This is so because the coefficient of friction is nearlyconstant in connection with most hard solid substances. However, effortsto define the friction properties of textile fibers by reference tocoefficients of friction have lead to disappointing results.

For most textile materials, the coefficient of friction is not aconstant. It varies somewhat irregularly as load changes and a number ofapparent inconsistencies have yet to be resolved. Moreover, the widerange of the valve of the coefficient of friction as a function of theload normal to the sliding surfaces greatly complicates attempts atanalysis of fiber friction forces upon the basis of coefficient offriction. Yet friction is important to textiles both from the standpointof the finished product and in connection with the problems ofprocessing the fibers.

SUMMARY OF THE INVENTION Accordingly, it an object of this invention toprovide a method and apparatus for determining a meaningful indicationof the frictional properties of fibers.

It is another object of the invention to provide a method and apparatusfor measuring the energy dissipated in the shear motion of a fiberaggregate.

It is a further object to provide a method and apparatus for determininginterfiber friction and internal structural stress characteristics offiber assemblies.

These objects may be realized by applying an oscillating shear loadingto a fiber aggregate and obtaining a measure of the energy transformedinto heat through fiber-to-fiber friction. According to one embodiment,the assembly of fibers is mounted between a stationary plate and a plateconnected to a pendulum. The pendulum then is set in motion and theamplitudes of successive oscillations are measured. The differencebetween adjacent amplitudes is a function of the work expended inovercoming friction, and a major portion of this friction isfiber-to-fiber friction in that instrument friction and internalfriction in the individual fibers are relatively small. A plot of thesquares of the differences in amplitudes of adjacent oscillationsagainst the sums of such amplitudes yields a substantially straightcurve the slope and intercept of which are statistically usablequantities characteristic of the fiber sample.

THE DRAWINGS The method and apparatus of the instant invention will bemore easily understood by reference to the accompanying drawings inwhich:

FIG. I is a perspective view of one embodiment of the apparatus forpracticing the invention in which a pendulum is used to impartoscillatory shear to a specimen of fibers;

FIG. 2 is a perspective view of a portion of the apparatus used tomeasure the amplitude of the oscillations of the pendulum of FIG. l; and

FIG. 3 is a graph showing typical curves obtained by the exercise of thepresent invention.

THE PREFERRED EMBODIMENT Referring to the drawings, a frame 110 ismounted on a base plate 12 of sufiicient weight to impart stability tothe frame Ml. An upright M of U-shaped horizontal cross section carriesa horizontal plate lib about 8 inches square and is movable within guidemeans I18 on an extension 20 of the frame 110. A screw 22 passes throughone wall of the guide means lltl through the open side of the U-shapedupright M, and on a threaded hole in a vertically extending scale member23 pivotally mounted on the base plate 112. A compression spring (notshown) surrounds the screw 22 between the wall of the guide Ill and thescale member 23 to urge the latter to the left (FIG. l) but the scale 23may be moved to the right by tightening the screw 22. The U-shapedupright M is adjustably positioned on the upper portion of the scalemember 23 and may be clamped in place thereon at the desired level bysuitable means 2A. A fiducial mark near the lower extremity of theupright M is thus disposed adjacent markings on the scale 23 calibratedso that the thickness of a fiber specimen 2b positioned on the plate llbmay be read directly.

An upper portion of the frame lltl carries a horizontal yoke 2% throughwhich a pendulum 30 passes freely and upon the upper surfaces of whichthe pendulum is supported through a pair of knife edges 32 rigidlyattached to the pendulum. When the pendulum is set in motion, it servesas a harmonic oscillator as energy is converted cyclically frompotential energy to kinetic energy and back again to potential energy.

Balanced on a second pair of knife edges 34 rigidly attached to theupper extremity of the pendulum Bill is a cross arm 36 having a flatplate 3% rigidly mounted at one end thereof. The plate 3% is of the samesize and configuration as the plate 16 and cooperates therewith inreceiving the assembly of fibers 2b, the frictional characteristics ofwhich are to be investigated. The cooperating surfaces of plates l6 and3ft may themselves be roughened or may, for example, be surfaced withsheets of sandpaper or the like so as to reduce the slippage between thespecimen of fibers under test and the plates 16 and 35%.

At the end of the cross arm 36 opposite the plate 3% there is suspendedan adjustable counterweight it) through which the cross arm 1% may bebalanced on the knife edges 341, and a guide d2 on the frame It)restricts the amount of permitted pivoting movement of the cross arm.After a fibrous sample 26 is placed between the plates 11.6 and 38, thelower plate to is moved upwardly to cause the sample to be contacted byboth of the plates 16 and 38. Then a standard weight 4141 of, forexample, one hundred grams is placed on the arm 36 above the plate 3% tocompress the sample, and the lower plate lid is moved upwardly again toposition the plate 33 in a horizontal plane. The leveling of the arm 36is greatly facilitated by the presence of a sensitive spirit level 46mounted on a midportion of the cross arm 3%.

If desired, a standard precompression treatment of the specimen may beaccomplished as an adjunct to the loading of the specimen. For somematerials, for example, it may be desirable to initially load the samplewith a 200 gram weight 44 for a period of time and then replace thatweight with a standard one hundred gram weight prior to the finalleveling operation.

In order that a standard initial amplitude of the oscillation of thependulum 3t) may be obtained, a latching device is carried by the framein position to cooperate with a hardened pin 47 on the lower end portionof the pendulum 3b. Although the specific construction of the latchingdevice is not of critical importance, there has been illustrated in FIG.11 a hardened hook 4d pivotally mounted at A9 on a cross member of theframe 110. Suitable spring and stop means (not illustrated) may beprovided for normally holding the hook 33 in a position to engage thepin A7. Then when the hook Aft is pressed downward, the pin 47 isreleased to permit free oscillation of the pendulum.

As shown more clearly in FIG. 2, a fan 50 having a hub 52 and a numberof equally spaced radial arms 5% is rotatably mounted on base plate I2beneath the pendulum hill. The fan may encompass less than or may bemounted at an angle to the vertical so that the pendulum 3h cooperateswith the fan 5% during displacement from the vertical in one directiononly. The upper surfaces 56 of the arms 54 may be curved slightlyconcave upward to correspond more nearly to the are through which thependulum 30 swings, and grooves 58 are provided in these upper surfaces.Low inertia slides 60 of paper or the like are mounted in the grooves 58for cooperation with a pusher element 61 on the lower end of thependulum 30.

The arrangement of the fan 50 with respect to the pendulum 30 is suchthat any one of the fan arms 54 may be brought into position under andin alignment with the path of the pusher element 61 on the lower end ofthe pendulum 30. As the pendulum swings, the pusher element contacts theslide or target 60 immediately therebeneath and moves such target to aposition corresponding to the outward extremity of the pendulums arc.The upper surface 56 of each of the fan arms 54 may conveniently beinscribed with a scale 62 so that the amplitude of the pendulum swingmay be read directly from the position of the slide 60.

In practicing the invention it is frequently convenient to employ as thespecimen 26 a stack of about one hundred 8-by-8 inch card webs forming aparallelepiped about inches thick. Such a fibrous assembly may bedeformed substantially upon vibration of the pendulum without beingdisrupted. That is to say, the movement of one face of the assemblyrelative to the other may be sufficient to cause a significant amount ofsliding motion of fibers relative to each other without breaking theassembly as a whole.

Although the dimensions of the components of the pendulum system may bevaried, it is desirable that the maximum amplitude of oscillation of theplate 38 be compatible with the type of specimen 26 employed. Forexample, a maximum amplitude of about one inch will be found to beentirely satisfactory in connection with a sample of the type describedin the preceding paragraph. In selecting dimensions, one should alsokeep in mind that the plate 38 should oscillate substantiallyhorizontally and that the amount of energy dissipated in the sampleshould significantly affect the amplitude of the pendulum 30.

In operation, the fiber assembly 26 is inserted between the surfaces ofthe plates 16 and 38 and the height of the lower plate 16 is adjusted bymoving the upright 14 in its guide 18 and relative to the scale member23. The weight 44 also may be applied at this time. When adjustment ofthe upright 14 has brought the plate 38 into a horizontal position (withthe pendulum 30 at rest and in a vertical position), thickness of thespecimen may be determined from the scale 23 at the lower end of upright14.

The pendulum 30 is then moved to the right in FIG. 1 toward frame intocooperative relation with the hook 48 by which the amplitude of theinitial oscillation is gauged. When the pendulum is released, it swingsto the left on its initial oscillation. During the portion of the swingof pendulum 30 to the left of the vertical, an arm 54 of fan 50 isrotated into alignment so that on the return swing to the right thepusher element 61 contacts a target or slide 60 and pushes it to alongthe arm 54 to register the maximum amplitude of that particular swing ofthe pendulum. As the pendulum 30 swings back to the left of thevertical, a second arm 54 of the fan 50 is rotated into alignment and onthe return swing of the pendulum 30 a second slide 60 is engaged andmoved by the pusher element 61 to record the extremity of that swing.The procedure is continued until the amplitudes of a number of swingshave been registered.

Each succeeding oscillation will have a smaller amplitude than itspredecessor. On each swing of the pendulum, the sample 26 is deformed,causing bending of the fibers and also fiber-to-fiber sliding atmultiple contact points. It is significant, however, that most of theenergy which goes into the bending of the fibers is not actually removedfrom the harmonically oscillating system but is in fact returned to thependulum during the reverse movement thereof. Thus, when the amplitudemeasurements are made with respect to complete "back and forth movementcycles, fiber bending affects the results only to the extent that a verysmall amount of work is consumed in internal friction within theindividual fibers. Instrument friction also represents a very smallfactor in the decay in amplitude of the oscillations of the pendulum,and for practical purposes changes in amplitude may be attributed tofiber-to-fiber friction within the sample 26.

A relation between change in amplitude and average friction force may bederived in the following manner.

As is well known, the energy of an oscillating pendulum may be expressedas n 1 n (n where A, is the amplitude of the n' vibration of thependulum and K is an instrumental constant. The difference in the energyof successive swings is then The distance D through which the upperplate 38 moves for these two successive oscillations of pendulum 30 maybe expressed as 2( n+ n+1) where K is the instrumental constant relatingthe arc through which pendulum 30 swings to the movement of plate 38.

Since the energy lost by the pendulum is also the average frictionalforce F multiplied by the distance D,

n n+ l 2( n+ n4-1) Equating (2) and (4), the average frictional forcemay be expressed as a( H n+1) where K =K,/4K

Although the amplitude data may be displayed in various ways, it hasbeen found particularly advantageous to plot the squares of thedifferences between successive amplitudes against the sums of successiveamplitudes, as indicated in FIG. 3. This type of curve was discovered tobe substantially straight. Hence, the slope and intercept of this curvewith the ordinate of the graph provide statistically usable values whichare characteristic of the frictional properties of the specimen undertest.

FIG. 3 shows curves representing values obtained from different fibers.It will be observed that the curves are well defined and that theydistinguish unequivocally from each other. These specimens rankthemselves in the order of their difficulty of hand-combing andprocessing.

Where large numbers of tests are to be made, it usually will be foundunnecessary to actually record a whole series of amplitude values foreach test and then to plot data on a chart as in FIG. 3. Practicallyuseful results can be obtained for example when only the amplitude ofthe fourth or fifth oscillation is recorded. A sufficiently closeapproximation of the slope valve for the particular test can be obtainedfrom the result of this measurement along because the amplitude of theinitial oscillation of the pendulum will be a constant established bythe apparatus configuration (e.g., position of hook 48, etc.

In many instances, it also is desirable to establish for the specimenbeing tested a characteristic value which is independent of thethickness of the specimen, that is, a value which would be substantiallythe same'for a plurality of specimens which differed from each otheronly in thickness. It has been discovered that such a value can beobtained by multiplying the specimen thickness, as measured on the scale23, by a number representative of the slope characteristic for thespecimen. For this purpose, it makes no fundamental difference whetherthe slope characteristic is determined from a curve such as that shownin FIG. 1 or from the numerical operation referred to in the precedingparagraph. For example, a highly useful and readily obtainable parameteris (A -M where A is the amplitude of the initial oscillation, A is theamplitude of the fifth oscillation of the pendulum, and T" is the squareroot of the thickness of the specimen.

Of course, where comparisons between samples are to be made, properattention must be given to possible differences in the samples and/ortest conditions. Different results may be obtained from tests on thesame fiber where there are differences in such factors as sample massand/or degree of compression, humidity, orientation of the fibers withinthe assembly, and the internal stress conditions of the fibers in thesample. For example, it has been found that the energy dissipated byoscillating a sample in shear is usually greater when the sample istested immediately after the carding operation than when the sample istested after there has been a sufficient time lapse during whichrelaxation of the stresses set up in the fibers by the carding operationmay occur.

The processing to which the fiber has been subjected prior to testing inaccordance with the invention may also have a significant effect on theresults. For example, a specimen of fibers that have been subjected torepeated carding operations may be expected to have energy dissipatingqualities different from a specimen made up of fibers that have beensubjected to less sever treatments.

Although a single embodiment has been illustrated and described indetail, various other embodiments will readily suggest themselves topersons skilled in the art. For example, the illustrated. pendulumsystem is obviously just one example of various harmonic oscillatorsthat may be used in carrying out the invention. Other modifications,such as different modes of evaluating the energy dissipated when thefibrous specimen is subjected to oscillating shear, will also beapparent. It is intended therefore that the foregoing be considered asexemplary only, the appended claims being intended to cover a broadrange of equivalent constructions and methods as within the true spiritand scope of the invention.

What is claimed is:

l. A method of testing interfiber frictional behavior in a unitary fiberassembly having spaced apart faces and within which frictionallycontacting fibers are capable of being moved relative to each otherwithout disrupting said assembly comprising:

moving one of said faces of the fiber assembly back and forth relativeto the other of said faces thereof in an amount and direction to loadsaid assembly in shear without disrupting said assembly, said back andforth movements causing bending of individual fibers within saidassembly and energy absorbing movements between frictionally contactingsurface portions of fibers within said assembly, and

obtaining a measure of the energy absorbed by the fiber assembly duringat least one back and forth movement of said one face relative to theother of said faces.

2. A method according to claim ll wherein the thickness of the fiberassembly is additionally measured.

3. A method of testing a unitary fiber assembly having spaced apartfaces and within which frictionally contacting fibers are capable ofbeing moved relative to each other without disrupting said assemblycomprising:

moving one of said faces of the fiber assembly back and forth relativeto the other of said faces thereof by an oscillating system in whichenergy is repetitively transformed from potential energy to kineticenergy and back again to potential energy, said movement being in anamount and direction to load said assembly in shear without disruptingsaid assembly and cause energy absorbing movement between frictionallycontacting fibers within said assembly, and

obtaining a measure of the energy absorbed by the fiber assembly duringat least one back and forth movement of said one face relative to theother of said faces.

4. A method according to claim 3 wherein said oscillating system is setin oscillation and then allowed to decay, and wherein amplitudes of aplurality of oscillations are measured to provide a measure of theenergy dissipated by reason of movements within the fiber assembly.

5. A method according to claim 4 wherein the squares of the differencesbetween successive amplitudes are plotted against the sums of successiveamplitudes to provide a generally straight curve the slope of which is astatistically usable quantity characteristic of the interfiber frictionproperties of the sample.

6. A method according to claim 3 wherein the amplitude of initialoscillation of the system is predetermined and wherein a measure of theamplitude of a later oscillation is recorded.

7. A method of testing a fibrous assembly made up of multiple card weblayers which comprises:

placing the fibrous assembly between a stationary horizontal surface andmovable horizontal surface connected to a pendulum;

imparting an oscillatory motion to said pendulum sufficient to causerelative back and forth movement of fibers within said assembly withoutdestroying said assembly; and

obtaining a measure of the amplitudes of a plurality of oscillations ofsaid pendulum.

fl. A method according to claim 7 wherein the amplitude of the initialoscillation of the pendulum is predetermined and wherein the amplitudeof a later oscillation is measured.

9. Apparatus for investigating properties of fibers comprisfirst andsecond surfaces disposed in a spaced parallel relation and being adaptedto receive therebetween a specimen of the fibers to be investigated,

means for imparting an oscillatory movement to one of said surfaces, and

means for measuring the decay in the amplitude of the oscillatorymovement of said one of said surfaces. fill. The apparatus of claim 9wherein said surfaces are pro vided with means for reducing slippagebetween said surfaces and the specimen.

ill. The apparatus of claim 9 including means for determining theparallel relationship of said surfaces.

H2. The apparatus of claim ll wherein said means for impart ing anoscillatory movement to one of said surfaces includes a pendulumconnected to said one of said surfaces.

llil. The apparatus of claim 112 wherein said means for measuring thedecay in the amplitude of the oscillatory movement of said one of saidsurfaces includes a record medium and means carried by said pendulum forrecording the maximum displacement of said pendulum from the vertical onat least one oscillation.

M. The apparatus according to claim ll? including means for measuringthe thickness of the specimen.

E5. The apparatus according to claim 14 including means for applying astandard compressive load to the specimen.

to. A method of testing interfiber frictional behavior in a specimenmade up of multiple card webs stacked together in a unitary assemblyhaving generally parallel external faces and an interior portion withinwhich frictionally contacting fibers are capable of being moved relativeto each other without disruption of the specimen as a whole comprising:

moving one of said faces back and forth relative to the other said facein a direction generally parallel to said faces to repetitively load theinterior portion of said specimen in shear and in an amount insufficientto disrupt said specimen, said back and forth movements causing bendingof individual fibers within said assembly and energy absorbing movementsbetween frictionally contacting surface portions of fibers within saidassembly, and

obtaining a measure of the energy absorbed by the specimen during atleast one back and forth movement of said one face relative to saidother face.

1. A method of testing interfiber frictional behavior in a unitary fiberassembly having spaced apart faces and within which frictionallycontacting fibers are capable of being moved relative to each otherwithout disrupting said assembly comprising: moving one of said faces ofthe fiber assembly back and forth relative to the other of said facesthereof in an amount and direction to load said assembly in shearwithout disrupting said assembly, said back and forth movements causingbending of individual fibers within said assembly and energy absorbingmovements between frictionally contacting surface portions of fiberswithin said assembly, and obtaining a measure of the energy absorbed bythe fiber assembly during at least one back and forth movement of saidone face relative to the other of said faces.
 2. A method according toclaim 1 wherein the thickness of the fiber assembly is additionallymeasured.
 3. A method of testing a unitary fiber assembly having spacedapart faces and within which frictionally contacting fibers are capableof being moved relative to each other without disrupting said assemblycomprising: moving one of said faces of the fiber assembly back andforth relative to the other of said faces thereof by an oscillatingsystem in which energy is repetitively transformed from potential energyto kinetic energy and back again to potential energy, said movementbeing in an amount and direction to load said assembly in shear withoutdisrupting said assembly and cause energy absorbing movement betweenfrictionally contacting fibers within said assembly, and obtaining ameasure of the energy absorbed by the fiber assembly during at least oneback and forth movement of said one face relative to the other of saidfaces.
 4. A method according to claim 3 wherein said oscillating systemis set in oscillation and then allowed to decay, and wherein amplitudesof a plurality of oscillations are measured to provide a measure of theenergy dissipated by reason of movements within the fiber assembly.
 5. Amethod according to claim 4 wherein the squares of the differencesbetween successive amplitudes are plotted against the sums of successiveamplitudes to provide a generally straight curve the slope of which is astatistically usable quantity characteristic of the interfiber frictionproperties of the sample.
 6. A method according to claim 3 wherein theamplitude of initial oscillation of the system is predetermined andwherein a measure of the amplitude of a later oscillation is recorded.7. A method of testing a fibrous assembly made up of multiple card weblayers which comprises: placing the fibrous assembly between astationary horizontal surface and movable horizontal surface connectedto a pendulum; imparting an oscillatory motion to said pendulumsufficient to cause relative back and forth movement of fibers withinsaid assembly without destroying said assembly; and obtaining a measureof the amplitudes of a plurality of oscillations of said pendulum.
 8. Amethod according to claim 7 wherein the amplitude of the initialoscillation of the pendulum is predetermined and wherein the amplitudeof a later oscillation is measured.
 9. Apparatus for investigatingproperties of fibers comprising: first and second surfaces disposed in aspaced parallel relation and being adapted to receive therebetween aspecimen of the fibers to be investigated, means for imparting anoscillatory movement to one of said surfaces, and means for measuringthe decay in the amplitude of the oscillatory movement of said one ofsaid surfaces.
 10. The apparatus of claim 9 wherein said surfaces areprovided with means for reducing slippage between said surfaces and thespecimen.
 11. The apparatus of claim 9 including means for determiningthe parallel relationship of said surfaces.
 12. The apparatus of claim 9wherein said means for imparting an oscillatory movement to one of saidsurfaces includes a pendulum connected to said one of said surfaces. 13.The apparatus of claim 12 wherein said means for measuring the decay inthe amplitude of the oscillatory movement of said one of said surfacesincludes a record medium and means carried by said pendulum forrecording the maximum displacement of said pendulum from the vertical onat least one oscillation.
 14. The apparatus according to claim 13including means for measuring the thickness of the specimen.
 15. Theapparatus according to claim 14 including means for applying a standardcompressive load to the specimen.
 16. A method of testing interfiberfrictional behavior in a specimen made up of multiple card webs stackedtogether in a unitary assembly having generally parallel external facesand an interior portion within which frictionally contacting fibers Arecapable of being moved relative to each other without disruption of thespecimen as a whole comprising: moving one of said faces back and forthrelative to the other said face in a direction generally parallel tosaid faces to repetitively load the interior portion of said specimen inshear and in an amount insufficient to disrupt said specimen, said backand forth movements causing bending of individual fibers within saidassembly and energy absorbing movements between frictionally contactingsurface portions of fibers within said assembly, and obtaining a measureof the energy absorbed by the specimen during at least one back andforth movement of said one face relative to said other face.